It's a well-known thing that actual speed is a floating point number and is rounded only on UI.

Another one well-known issue is that floating point calculations are not precise (sometimes your get a bit more, sometimes a bit less than expected)

I've done an experiment.

I took two champs with exactly 100 base speed. Then put a single art with +12 SPD on one of them and a single Speed Set with +12% SPD (without flat speed) on another one. I expected two equal champs with 112 speed, but the flat one always goes first. No matter what order they have in a team (the order does matter with equal speed). Same results with Perception set (+5%).

But with floating point numbers

100 * 0.12 is exactly 12

and

100 * 1.12 is greater than 112

Now I suspect that percentage bonus is always lowered. Just a bit, so I had like 111.999999 speed in my tests. But against a flat-geared guy on the arena it would work like a whole -1 speed.

It would change my considerations when gearing.

Can anyone confirm that it happens with all speed gaining sets?